This invention relates to reference electrodes and ion sensors of solid type, which have a polymer film covering a surface of an electrically conductive substrate, and also methods of their manufacture.
Heretofore, as electrochemical reference electrode is selected one, the output potential which is not substantially changed electrochemically while it is in a liquid under test, which is subject to changes in the ion concentration, particularly hydrogen ion concentration. Well-known examples of such reference electrode are typically saturated calomel electrodes, silver/silver chloride electrodes and hydrogen electrodes. Further, recently researches and investigations are being conducted on commonly termed solid type electrodes, having a film of an organic substance such as polystyrene and parylene, on a gate insulation film of a MOS (metal oxide semiconductor) field-effect transistor (hereinafter referred to as MOS FET).
However, the prior art reference electrodes such as saturated calomel electrodes can be miniaturized with difficulty because there is a liquid junction between reference electrolytic solution and a vessel therefor.
As a method of forming an organic film of polystyrene or the like for a reference electrode, there is a plasma polymerization method (as disclosed in Japanese Patent Disclosures 58-103658 and 58-34352).
Meanwhile, as well-known ion sensors there are ones, in which a carbon layer is formed on an insulating layer (for instance of Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5), one, in which a redox function film (for instance a polymer layer of 1-pyrenamine, 2, 6-dimethylphenol or 4, 4'-biphenol) is formed by an electrolytic polymerization process on an electrically conductive carbonaceous material, and one, in which an ion-sensitive film is formed to cover the redox function film noted above. In order for miniaturization of even these ion sensors, it is proposed to form a structure consisting of a carbon layer, an oxidizing /reducing function film and an ion-sensitive film on a MOS FET gate insulation film or to cover an extended gate electrode surface (as disclosed in Japanese Patent Disclosures 63-131056 and 62-276452).
However, with reference electrodes formed on MOS FET by the plasma polymerization process noted above, the stability of output potential is insufficient because of passage of ion active substance through the polystyrene film mainly due to swelling of the film and also of influence of active residue or functional radicals produced during the electrolytic polymerization.
Further, in an ion sensor formed on MOS FET, the carbon layer formed on the insulating layer is weak in adhesion and mechanical strength and also greatly differs in the expansion coefficients and lattice constants from the redox function film. Therefore, the redox function film is liable to be separated together with the carbon layer from the insulating layer during or after the electrolytic reaction. If the carbon layer is made very thin, for instance below 1,000 angstroms in thickness, although the adhesion is increased, the electric resistance is extremely increased (to 100 k.OMEGA./.quadrature. or above). In such a case, electrolytic reaction can be caused with difficultly. Further, where a coating process or dipping process is used, a chemically obtained polymer can not provide a sufficient function, and a stable potential can not be obtained. For this reason, it is necessary to use a redox function film which is prepared by the electrolytic polymerization process. This film however, is difficultly soluble and can not find an adequate solvent.